1. Technical Field
The present invention relates generally to satellite navigation technology and, more particularly, to a scheme for tracking a binary offset carrier (BOC) signal that is used for satellite navigation technology.
2. Description of the Related Art
Satellite navigation technology is technology that is configured such that when a plurality of navigation satellites randomly transmit a plurality of satellite navigation signals, each containing information about the current location and time of the corresponding navigation satellite, to the ground, a satellite navigation receiver on the ground receives the plurality of satellite navigation signals, calculates the coordinates of the current locations of the navigation satellites and the arrival times of the signals, and determines its three-dimensional (3D) location in the Earth coordinate system using triangulation.
A satellite navigation receiver theoretically requires at least three satellite signals in order to determine its longitude, latitude and height, and requires one more satellite signal in order to improve accuracy by eliminating time error between satellites. Accordingly, at least four satellites are required.
Across the world, many countries have developed independent satellite navigation systems for economic and military reasons. Although the United States Global Positioning System (GPS) is most widely used and famous, the European Union's Galileo system, the Russian GLONASS, the Chinese COMPASS system, and the Japanese QZSS system (which will be expanded to the JRANS system in the future) are also being currently operated or developed.
Since satellite navigation signals should be robust to interference and jamming, various and elaborate modulation schemes have been employed. It is worthy of notice that the majority of the next-generation satellite navigation systems have replaced a conventional a phase shift keying (PSK) modulation scheme or have employed a BOC modulation scheme in addition to a PSK modulation scheme. The width of the main peak of an auto-correlation function used for the BOC modulation scheme is short, and thus the BOC modulation scheme exhibits better signal tracking performance than the PSK modulation scheme.
Furthermore, the BOC modulation scheme is characterized in that spectral separation occurs and energy is shifted from the center of a band to the periphery thereof, unlike the PSK modulation scheme, and thus the BOC modulation scheme can be additionally applied to a band in which a conventional modulation scheme has been used. Using these characteristics, the next-generation satellite navigation systems can employ the BOC modulation scheme in addition to the PSK modulation scheme, thereby being able to ensure the improvement of performance and backward compatibility.
A BOC signal is a signal that is expressed as a product of a pseudo random noise (PRN) code with a sine or cosine rectangular sub-carrier. A BOC signal is expressed as a BOCsin(kn,n) or a BOCcos(kn,n) depending on the type of sub-carrier, where k is a positive integer indicative of the ratio of the chip period of a PRN code to the period of a sub-carrier, and n is indicative of the ratio of PRN code chip transmission rate to 1.023 MHz, that is, the clock frequency of a CA code.
Although a BOC signal has high signal tracking performance and excellent compatibility with the conventional PSK modulation scheme, it is problematic in that many side peaks occur around a main peak where an auto-correlation function has the highest value, unlike the PSK scheme having a single peak. A problem in which, upon tracking a BOC signal, synchronization is established with a side peak instead of a main peak due to the presence of side peaks, that is, the so-called ambiguity problem, may occur.
Meanwhile, in order to modernize the GPS system while maintaining its backward compatibility and provide compatibility between the GPS system and the Galileo system, a multiplexed BOC(MBOC) modulation method was proposed, and the U.S. and European authorities finally decided to adopt a so-called MBOC(6,1,111) modulation method in which a BOCsin(1,1) signal and a BOCsin(6,1) signal were combined at a power split ratio of 1/11 after discussion.
Interestingly, the U.S. and European authorities implemented different methods of synthesizing sub-carrier signals BOC(1,1) and BOC(6,1) that could satisfy the power spectrum density of the MBOC(6,1,111) modulation method. First, the U.S. authority implemented a time-multiplexed BOC (TMBOC) using two sub-carriers BOC(1,1) and BOC(6,1) in the time domain in an non-overlap manner. In contrast, the European authority implemented a composite BOC(CBOC) in which a sub-carrier BOC(6,1) has been added to a sub-carrier BOC(1,1) along the time axis.
Meanwhile, the TMBOC modulated signal is configured such that 75% of power is assigned to pilot components and 25% of the power is assigned to data components, thereby allowing BOC(1,1) spreading symbols to be used for data having no advantage attributable to high sub-carrier frequency. In contrast, the pilot components are configured to include 29/33 BOC(1,1) spreading symbols and 4/33 BOC(6,1) spreading symbols, and thus an advantage attributable to the high frequency of a sub-carrier can be obtained via pilot components and also signal tracking performance can be improved.
Furthermore, the pilot components of the TMBOC modulated signal are stipulated such that BOC(6,1) spreading symbols are located at the four chip locations of the Nos. 1, 5, 7 and 30 chips of every 33 chips and BOC(1,1) spreading symbols are located at the remaining 29 chip locations (which is expressed as a TMBOC(6,1,4/33)). This 33 chip pattern is repeated 310 times, and thus a spreading code has a 10230 chip length.
The TMBOC(6,1,4/33) signal includes BOCsin(6,1) signal components at the ratio of 4 chips to 33 chips. Since the main peak of the auto-correlation function of the BOCsin(6,1) signal components is sharper than the main peak of the auto-correlation function of the BOCsin(1,1) signal component, more excellent positioning accuracy can be provided.
However, the TMBOC(6,1,4/33) signal still has side peaks around the main peak of an auto-correlation function, and thus the ambiguity problem still remains.